Unpleasant odor from tube amp

[KenC]

I have a couple of questions for the amp gurus. Last weekend I put the B-15 away and pulled out my old '82 Fender Bassman Ten. I had the original tubes and some caps in that amp replaced about 18 months ago, and it has been stored away for a couple of months (since the B-15 arrived). I used the Bassman Ten for a jam session last Sunday, and it sounded as great as ever. I played it a couple of times last week, and again all was well. When I powered it up on Friday, I noticed a very foul odor. I'd compare it to the odor you get from dust burning off heater coils when a furnace is turned on for the first time in the fall. There was also a hint of a burning rubber smell. There was no other evidence of anything burning, and nothing felt hot except the power tubes (Tung Sol 6L6GCs). I turned it on again today, and after a couple of minutes the odor returned. Again, there was no sign of anything overheating. The Bassman Ten is a tube amp with a solid-state rectifier, and is entirely point-to-point wiring. The power tubes may have been glowing a bit more than normal, and they both have black buildup (or scorching) on the side facing away from the amp. I'm not 100% certain that it wasn't a cable problem, but I lost volume briefly today. I'm not sure that the volume has been entirely consistent over past couple of days, either. FWIW, the odor seems to be stronger near the power tubes than anywhere else along the chassis.

My questions:

1. Is this a sign that the power tubes are at the end of their life cycle?

2. If my last experience with the local amp technician wasn't very positive, would there be any reason not to buy a matched pair of tubes (probably NOS) and install them myself?

3. Any other ideas what could be going on?

Thanks in advance!

Ken

[KenC]

A bit more:

I just turned it on again for a couple of minutes. When I turned it off (as I always do - standby switch first, then power), the "power on" light stayed lit. I flipped the switches back to the "on" position and then off again, and the light went out. I recall this happening when I got the amp back from the technician the first time after the retube/recap (it took three trips back to the shop to get it working properly - which is why I hesitate to see a technician without having my ducks in a row first).

Ken

[suave eddie]

What you smell is "mojo".


(Sorry, I can't really help)

[meursault]

if you want to change the power tubes by yourself, make sure you can bias the damn thing by yourself as well...

[darwinohm]

Ken, you appear to be able to pull the chassis out and power it up, (make sure the speakers are plugged in), without killing yourself. If uncertain of that, don't do it! I suspect you will discover the source of the odor within a couple of minutes. Once you see what is smoking or smelling, power it off and find a copy of the schematic or get one of the Fender Web Site. Anywhere you touch something in the chassis, be sire to discharge it to ground with a long, plastic or rubber handled screwdriver or you may glow. The part that is smoking or smelling may likely be caused by something else drawing too much current. I had a similar situation with my Custom 15 and found a bad fuse holder, the fuse had overheated and burned the circuit board. I hand wired in a circuit to bypass the fuse circuit. It still works fine to this day. The fuse clips did not have enough tension, overheated, and the clips lost their spring tension until it smelled and finally blew the fuse. I have used the smoke test more than once to solve a problem. You are lucky that it is hand wired. It is fixable compared to buying a new pc board -- Darwin

[KenC]

Thanks for the quick replies! I think the best course of action will be to find a good technician. I won't be going back to the same guy as before. Time to lug this one downstairs and the B-15 back up...

Ken

[Elwood]

Time to lug this one downstairs and the B-15 *back up...

Ken

Good to have a great back up ( is that why they call it a "*back up" ? ) .

It sounds like some of the magic smoke has been released.
A good tech should be able to troubleshoot that with not a lot of bench time I imagine.

[NickHorne]

Passing this issue on to a good tech has to be the right answer, if in any doubt at all.
An amp that has been switched off and unplugged from the wall can still kill, sometimes many hours later, unless discharging is done just right.

[KenC]

Passing this issue on to a good tech has to be the right answer, if in any doubt at all.
An amp that has been switched off and unplugged from the wall can still kill, sometimes many hours later, unless discharging is done just right.

Thank you for that reminder, Nick! The Bassman is in the garage waiting for a trip to a good tech (whenever I find one), and the JC-120 that's been collecting dust since I moved into this house three years ago is getting some love.

I have mentioned in several posts that my JC-120 sounded great with small MFDs, but that an ASAT sounded way too shrill through it. I have to take that statement back. My previous experience pairing an ASAT with this amp was with an '88 that I no longer own. I ran my '86 through it tonight, and all I can say is WOW. I'm looking forward to getting reacquainted with the Jazz Chorus, and might even need to rearrange the music room so it can fit beside one of the bass amps.

Ken